1. Field of the Invention
This invention relates to high temperature heating vessels, and in particular, to technique for prolonging the service life of an lid for a glass batch melting furnace by protecting the inner exposed surface of the lid.
2. Technical Considerations
One type of glass melting process entails feeding of glass batch materials onto a pool of molten glass contained in a tank type melting furnace and applying thermal energy to melt the materials into the pool of molten glass. The melting tank conventionally contains a relatively large volume of molten glass so as to provide sufficient residence time for currents in the molten glass to effect some degree of homogenization before the glass is discharged to a forming operation
U.S. Pat. No. 4,381,934 to Kunkle and Matesa, which teachings are incorporated by reference, discloses an alternative type of glass melting arrangement, and more particularly an intensified batch liquefaction process in which large volumes of glass batch materials are efficiently liquefied in a relatively small liquefaction vessel. This type of process, particularly when using intensified heat sources such as oxygen flame burners, produces relatively small volumes of high temperature exhaust gases.
During the heating and melting process, it is believed that certain components of the batch material vaporize. These vapors may be corrosive to exposed metal and refractory surfaces and when combined with the hot exhaust gas stream that circulates through vessels of the type disclosed in U.S. Pat. No. 4,381,934, corrode exposed interior surfaces, and in particular the vessel lid. In addition, the exhaust gas may entrain particulate matter within the vessel which may act as an abrasive on an exposed surface. This corrosive and abrasive gas stream greatly reduces the service life of the vessel lid which may result in increased costs and additional down time for lid repair and replacement.
The high temperatures within the vessel may also pose additional processing problems. For example, heat loss will effect the efficiency of the operation. The more heat that is lost during the liquefaction process through uninsulated and/or exposed interior surfaces of the vessel, the less efficient the liquefaction process becomes. This may require additional heat input to the vessel in order to account for the amount of heat lost. In particular, the removal of heat by cooling the vessel lid in order to reduce heat degradation and prolong service life reduces the overall heating efficiency of the operation. If this heat loss could be controlled and reduced, the overall efficiency of the operation would be increased.
It would be advantageous to have a heating vessel lid with a protective coating on its exposed inner surface that both insulates the lid, thus reducing heat loss from the heating vessel, and protects the exposed inner surface from a high temperature corrosive gas stream entrained with abrasive particulates, so as to increase its service life and decrease overall operating costs.
U.S. Pat. No. 3,165,301 to Riviere teaches a method and device for protecting refractory walls. A burner positioned in the roof of an elongated horizontal furnace flows a gaseous suspension of carbon particles along the roof to protect the roof against heat radiating from the flame formed by burners in the furnace. The carbon particle suspension is circulated within the furnace parallel to the roof and in a direction opposite to that of the main burner flame. The arrangement requires additional gas to be added to the heating system. Furthermore, the carbon particles are an additional contaminant in the heating operation.
U.S. Pat. No. 4,021,603 to Nanjyo et al teaches a cooled metal roof assembly for an arc furnace with refractory liner to protect the interior roof surface from high heat. Fire brick or other refractory material is provided within grooves formed on the interior surface of the roof to improve resistance to heat of the furnace roof assembly. The refractory material must be periodically replaced in order to ensure proper thermal insulation. In addition, the center portion of the lid is a consumable substructure that includes three hole for electrodes. There is no protection provided to this portion of the lid.
U.S. Pat. No. 4,182,610 to Mizuno et al teaches a water cooled metal cover for steel making or smelting furnace. Fins in the form of a lattice structure extend from the interior surface of an annular portion of the cover to provide a surface to which slag resulting from splashes within the furnace may adhere. The splashes of slag that adhere to the fins insulate the lower surface of the cover's cooling jacket. The center portion of the cover which includes openings for electrodes, does not have the lattice structure to accumulate the slag so that there is no protection provide on this portion of the lid. In addition, the random splashing of the slag does not provide a uniform buildup of insulating material over the entire cover surface.
U.S. Pat. No. 4,434,495 to Tomizawa et al. teaches a cooling pipe structure for arc furnaces. Wherein cooling pipes are embedded within refractory blocks. The pipes are positioned adjacent to the surface of the block facing the inside of the furnace to intensify the cooling of the surface. Slag plashed against the block surface will congeal and adhere to the block to form an insulating film.
The prior teaches lid construction furnaces but does not disclose controlling the cooling of the lid to permit materials entrained in hot gases circulating within the furnace to be deposited on the inner surface of the lid to form a relatively uniform and continuous insulating and protective layer herein thickness of the layer, and its associated insulative properties may be adjusted by varying the cooling of the lid.